High-gain diode-pumped laser amplifier

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of patent application Ser. No. 10/998,268 filed Nov. 26, 2004, now U.S. Pat. No. 7,590,160.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to solid state laser amplifiers pumped by semiconductor laser diodes and, in particular, to a laser amplifier operating using a wedge configured laser active slab to reduce the generation of amplified spontaneous emission within the laser amplifier.

2. Description of the Background Art

It is known in the relevant art to use laser emission from one or more semiconductor diode lasers to pump a solid-state laser medium (i.e., diode pumping). Diode pumping provides for high power solid-state laser devices that are more efficient, more compact, more reliable, and that have better beam quality, than lamp-pumped solid-state lasers of comparable output power. Many new diode-pumped solid-state laser (DPSSL) designs have emerged in recent years due to an increasing array of new solid-state laser materials, and a wider range of diode laser pump wavelengths from which the laser designer can select.

Various types of semiconductor diode lasers have become popular for pumping solid-state lasers. These include discrete single-emitter diode lasers as well as one-dimensional (1-D) diode laser arrays (i.e., diode “bars”) in which multiple diode laser emitters are integrated onto a single bar of semiconductor material. Two-dimensional (2-D) laser diode arrays incorporate multiple diode laser bars, packaged one above the other, to create a 2-D array of diode laser emitters. Since the emission of the individual diode emitters produce relatively large beam divergence, packaged diode lasers, diode bars, and 2-D diode arrays may include micro-optics to collimate one or both axes of the spatial emission pattern from each diode emitter. In some applications, it may be desirable to couple diode pump light into one or more optical fibers for delivering pump light to the solid-state laser medium.

Diode-pumped solid-state lasers are conventionally categorized as being either end-pumped or side pumped lasers. In end-pumped laser designs, diode laser radiation used for optical pumping travels substantially parallel to the laser beam being generated, or amplified, in the solid-state gain medium. In side-pumped laser designs, diode laser pump radiation travels substantially perpendicularly, or orthogonal, to the laser beam being generated or amplified. End-pumped DPSSLs are typically very efficient (e.g., slope efficiency may be 50% or more), and exhibit very good TEM₀₀(diffraction-limited) beam quality. End-pumped lasers designs are generally regarded as being limited to generating relatively low average output power levels, due to the possibility of thermal fracture of the solid-state laser medium when diode pump power is increased beyond a specified level. End-pumped DPSSL designs that allow pump light to penetrate deeply into the solid-state medium, and that distribute pump power more uniformly along the entire length of a laser rod or slab, are exceptions, but these lasers often suffer from degraded output beam quality unless certain preventive design measures are implemented.

In contrast, side-pumping methods enable the pumping of solid-state laser media with much higher power levels before thermal fracture occurs, mainly because the diode pump power is distributed over a larger area of the solid-state crystal surface. Historically, side-pumped DPSSL designs have been somewhat less efficient, and have exhibited poorer beam quality, than end-pumped lasers. Although side-pumping of round cross-section laser rods is possible, many side-pumping schemes utilize a rectangular slab of laser active solid-state material, where the laser active slab has an either square or rectangular cross section. Laser active slab geometries may provide for heat removal from the laser medium such that the thermal gradient established by the heat removal occurs primarily in a single direction. This configuration allows a linearly-polarized laser beam to be amplified in the laser active slab, with the polarization of the laser beam either parallel or perpendicular to the thermal gradient, and without objectionable effects due to thermal stress-induced birefringence.

Several diode-side-pumped laser gain module, or amplifier, schemes have emerged in recent years that enable matching of a TEM₀₀laser beam or resonator mode to the optically pumped volume of the solid-state laser medium, as is typically required for efficient operation. For example, Eggleston et al. [“The slab geometry laser—Part I: Theory,” <i>J Quantum Electronics</i>, vol. 20, pp. 289-301 (1984)] describe a side-pumped zig-zag slab scheme in which the beam being amplified is reflected multiple times, via total internal reflection (TIR), at each of two parallel faces of the slab. Diode pump power is injected into the slab through the same two faces configured for reflecting the amplified beam via TIR. These two surfaces are also used for cooling the slab. A zig-zag path taken by the amplified beam helps to average out, or mitigate, spatial distortion effects on the amplified beam profile that pump-induced and cooling-induced non-uniformities in the slab might otherwise have.

As taught by the Eggleston reference, the same two parallel faces of the laser active slab are used for pumping through, heat removal, and reflecting the amplified beam in the laser active slab. These requirements significantly complicate the design and fabrication of such laser active slab designs. While the use of TIR to establish a zig-zag beam path helps to prevent amplified spontaneous emission (ASE) and parasitic oscillation problems, it also limits the number of reflections, and therefore the single-pass gain length, the designer can achieve along a given length of laser active slab medium.

There is also disclosed in U.S. Pat. No. 5,271,031, issued to Baer, a zig-zag slab design in which diode pump light is injected through the same parallel slab side faces used for guiding an amplified beam along a zig-zag path in a laser active slab. Baer '031 further teaches a high-efficiency, mode-matched, solid-state laser with transverse pumping and cascaded amplifier stages. Thin-film dichroic optical coatings, highly reflecting at the laser wavelength and highly-transmitting at the diode pump wavelength, are disposed on the parallel lateral faces. TIR was not used in establishing a zig-zag beam path through the laser active slab. The slab was cooled through transverse faces (i.e., top and/or bottom surfaces of the laser active slab), and not through the lateral faces used for diode pumping and zig-zag beam reflection.

In the Baer '031 design, a single 1-D array diode bar pump source for pumping through one lateral face, or two diode bars for pumping through both parallel lateral faces, are positioned very close (about 0.45 mm) to the laser active slab so as to minimize divergence of the emitted beams from the individual emitters in the diode bar (i.e., beamlets) before entering the laser active slab medium. This configuration provides for distinct individual beamlets that enter and pump the laser active slab material, and thus produce a corresponding 1-D array of discrete gain regions in the lateral side of the laser active slab. The array of diode-pumped gain regions is in a one-to-one correspondence with the pattern of diode emitters in the diode bar pump source, and has the same spacing (or pitch) between gain regions as the pitch of the diode emitters on the laser bar. That is, for a uniformly spaced array of diode emitters, the array of discrete gain regions in the slab is likewise uniformly spaced with the same pitch.

The apexes of a zig-zag path taken by an amplified beam through the laser active slab are aligned to spatially overlap with all of the diode-pumped gain regions corresponding to the emitter positions of the diode laser bar. With this “tightly folded” configuration, Baer was able to demonstrate a laser amplifier gain as high as 15 dB (i.e., an amplification factor of 32) for a single pass through the laser amplifier, when pumping the laser active slab with a 10 W diode bar. However, using the design taught by Baer '031 presents a challenge in both initially achieving and maintaining the tightly-folded beam alignment with the diode emitters. This complicates practical applications of the Baer '031 design, especially when applied to a moderate- or high-volume production setting. Moreover, unwanted amplified spontaneous emission (ASE), or parasitic oscillation, generated at the lasing wavelength of the laser active medium can build up in the laser active slab between the parallel high-reflection lateral surfaces. This unwanted ASE generation can dramatically reduce amplifier gain unless certain special precautions are taken for prevention of the parasitic oscillations and ASE.

Several methods are described in Baer '031 for preventing parasitic lateral oscillations and ASE. One method involves producing micro-patterned coatings on the parallel faces of the laser active slab, with the coatings having alternating high-reflection (HR) and anti-reflection (AR) coating regions. During operation of the laser amplifier, a zig-zag beam path reflects off the parallel sides of the laser active slab at the HR regions of the coatings. The HR regions have the same pitch along the length of the laser active slab as the diode emitter pitch along the length of the diode bar. An HR region on one of the parallel faces of the laser active slab is directly opposite an AR region on the other parallel face, thereby preventing, in theory, the buildup of lateral parasitic oscillations or ASE across the width of the laser active slab. Another method for suppressing lateral parasitics described in Baer '031 involves coating both parallel surfaces with an HR coating, and then selectively etching away the coating so that an HR region on one of the parallel faces is directly opposite a region on the other parallel face in which the HR coating has been etched away. Such micro-patterned coatings are difficult and expensive to fabricate, and may not be considered practical for production in moderate or high volume applications.

A third method for suppressing lateral parasitics, described in Baer '031, is to “slightly wedge” the two nominally parallel faces. The reference does not quantify the magnitude of the wedge angle except to state that, if the wedge angle is too large, then a diode bar with non-uniform spacing between the diode emitters is needed to maintain mode matching between diode emitter positions and apex positions of the zig-zag beam path. This is because the reflecting surfaces would no longer be parallel, and the apex points of the zig-zag path would therefore be spaced non-uniformly along the length of the laser active slab. In a scientific article related to Baer '031, [Baer T M, et al., “Performance of diode-pumped Nd:YAG and Nd:YLF lasers in a tightly folded resonator configuration,” <i>J Quantum Electronics</i>, vol. 28, pp. 1131-1139 (1992)], a wedge angle of 0.6 milliradians (i.e., 0.035 degrees or 2 arc-minutes) was mentioned as “probably” adequate to suppress lateral parasitics, but small enough to maintain mode matching at uniformly spaced diode emitter positions. However, it has been demonstrated that the 0.6 milliradian lateral wedge angle cited in Baer '031 is not adequate to suppress lateral parasitics and ASE when pumping Nd-doped 1064-nm laser slabs with high power levels of 40 W or more.

The shortcomings of the design taught by Baer '031 are also noted in U.S. Pat. No. 5,651,021 issued to Richard et al. Richard et al. '021 observe that, in the tightly folded design taught by Baer '031, “the maximum power obtainable from the design is limited by super-radiance (ASE) occurring between the parallel opposite reflective coatings. Furthermore, the multiple-pass optical path is quite complicated due to the requirement to match the reflection points to the active areas of the laser diode”.

Richard et al. '021 disclose a diode-side-pumped zig-zag slab laser design that employs a five-sided slab. In this design, total internal reflection (TIR) is used to guide the beam or mode being amplified along a zig-zag path that makes two passes along the length of the laser active slab. The laser active slab is diode-pumped through the two longest (and parallel) sides of the laser active slab, which also reflect the zig-zag beam path. A third side of the laser active slab, oriented perpendicularly to the two parallel long sides, provides another TIR reflection that sends the beam back through the laser active slab for a second zig-zag pass, reflecting again from the parallel lateral sides. Two Brewster-angle faces at one end of the laser active slab provide entrance and exit facets for the laser beam or mode being amplified. An overall two-pass, or round trip, gain length of about 76 mm is achieved with a laser active slab that is about 15 mm long and 3 mm wide, according to a related paper by Richard and McInnes [“Versatile, efficient, diode-pumped miniature slab laser,” <i>Optics Letters</i>, vol. 20, pp. 371-373 (1995)].

The slab in the design disclosed in Richard et al. '021 is relatively easy and inexpensive to fabricate because it has no coated or curved surfaces. Lateral parasitic oscillation and ASE are prevented by virtue of using TIR to create the zig-zag beam path through the laser active slab. Although a 76 mm of round trip gain length for a 3 mm wide by 15 mm long laser active slab is advantageous, such gain length may be substantially shorter than what can be achieved if, for example, high-reflection coatings and steeper incidence angles at the reflecting faces are used to confine the two-pass zig-zag beam path in the laser active slab.

U.S. Pat. No. 5,774,489 issued to Moulton et al. discloses a diode-side-pumped laser active slab amplifier in which the beam being amplified enters and exits through the longitudinal end faces of the laser active slab. The beam being amplified does not reflect off the lateral side surfaces through which diode pump light is injected. Rather, the amplified beam is directed with mirrors, positioned at the longitudinal ends of the laser active slab, to make multiple zig-zag passes through the laser active slab along its greatest dimension. The end mirrors used to achieve the zig-zag beam configuration may be external mirrors, or, alternatively, end mirrors may be coated directly onto a portion of each end face of the laser active slab, leaving window sections on end faces for the amplified beam to enter and exit.

As taught by Moulton et al. '489, longitudinal parasitic oscillations or ASE can build up between the nominally parallel end mirror surfaces used to achieve the zig-zag beam configuration. Furthermore, this and other gain module designs in which the amplified beam enters and exits the laser active slab at the end faces are difficult to multiple-pass without using a Faraday rotator. Also, because of the multiple segmented coatings that may be required, the laser active slab design taught by Moulton et al. '489 can be difficult and expensive to fabricate, especially if the end mirrors used for creating the zig-zag beam path are coated directly onto the slab end faces.

It can be appreciated by one skilled in the relevant art that most prior art devices that use a zig-zag slab laser design employ total internal reflection (TIR) at parallel side faces to establish a zig-zag beam path through the laser active slab. Because the internal angle of incidence of the zig-zag beam path at the reflecting surfaces must be larger than a specified minimum angle, a TIR design limits the overall length of the zig-zag beam path and, therefore, the laser gain-per-pass that can be achieved in the gain module or amplifier. Designs that employ thin-film coatings to establish a zig-zag path through the laser slab enable steeper angles of incidence, more reflections, longer zig-zag path lengths and, therefore, potentially higher amplifier gain-per-pass, but only if build up of unwanted parasitic oscillations and amplified spontaneous emission (ASE) are prevented. It does not appear that prior art devices employing thin-film coatings to establish a zig-zag beam path have successfully dealt with the problem of unwanted parasitics and ASE. In addition, for some prior art laser amplifier devices using a two-pass amplifier configuration to increase overall gain, especially for devices that employ thin-film coatings, a Faraday isolator or rotator device must be included to separate the input and two-pass output beams. Furthermore, achieving three or four zig-zag passes in such thin-film-coated slabs has often not been possible or practical.

SUMMARY OF THE INVENTION

The disclosed apparatus and method utilize a laser active slab with a source of pump power to amplify an input laser beam, where the laser active slab includes a block of laser active material having opposed lateral faces defining a wedge lateral dihedral angle, opposed longitudinal faces, and opposed parallel transverse faces. The wedge lateral dihedral angle may be selected so as to minimize parasitic amplified spontaneous emission in the laser active slab. The lateral faces may include optical coatings highly transmitting at a wavelength of the pump power and highly reflecting at a lasing wavelength. The source of pump power may be one or more laser diode bars and microlenses producing a gain sheet in the laser active slab. The apparatus may include one or more external mirrors highly reflecting at the lasing wavelength positioned and oriented to provide additional zig-zag passes through the gain sheet for the input laser beam and thereby providing multi-pass laser amplifiers.

DESCRIPTION OF THE DRAWINGS

<figref idref="DRAWINGS">FIG. 1</figref> is a diagrammatical representation of a laser amplifier including an input laser beam, a laser active slab, diode bars, and microlenses, in accordance with the present invention;

<figref idref="DRAWINGS">FIG. 2</figref> is an end view of the laser amplifier of <figref idref="DRAWINGS">FIG. 1</figref> showing cooling surfaces;

<figref idref="DRAWINGS">FIG. 3</figref> is an alternate configuration of the laser amplifier of <figref idref="DRAWINGS">FIG. 1</figref> showing diode bars oriented at an angle at the laser active slab;

<figref idref="DRAWINGS">FIG. 4</figref> is a functional diagram of the laser amplifier of <figref idref="DRAWINGS">FIG. 1</figref> illustrating a single pass operational mode;

<figref idref="DRAWINGS">FIG. 5</figref> is a detail diagrammatical view of the laser active slab of <figref idref="DRAWINGS">FIG. 1</figref> illustrating a lateral dihedral angle feature;

<figref idref="DRAWINGS">FIG. 6</figref> is a diagrammatical view illustrating removal of parasitic oscillation from the laser active slab of <figref idref="DRAWINGS">FIG. 5</figref>;

<figref idref="DRAWINGS">FIG. 7</figref> is a detail diagrammatical view of the laser active slab of <figref idref="DRAWINGS">FIG. 1</figref> illustrating an alternative lateral dihedral angle feature;

<figref idref="DRAWINGS">FIG. 8</figref> is a side view of the laser active slab of <figref idref="DRAWINGS">FIG. 7</figref> showing a gain sheet;

<figref idref="DRAWINGS">FIG. 9</figref> is a graph of relative gain as a function of lateral displacement along the laser active slab of <figref idref="DRAWINGS">FIG. 1</figref> for different levels of pump energy absorption;

<figref idref="DRAWINGS">FIG. 10</figref> illustrates a single-pass mode of operation of the laser amplifier of <figref idref="DRAWINGS">FIG. 1</figref> for a first angle of incidence of the input laser beam.

<figref idref="DRAWINGS">FIG. 11</figref> illustrates a single-pass mode of operation of the laser amplifier of <figref idref="DRAWINGS">FIG. 1</figref> for a second angle of incidence of the input laser beam.

<figref idref="DRAWINGS">FIG. 12</figref> illustrates a single-pass mode of operation of the laser amplifier of <figref idref="DRAWINGS">FIG. 1</figref> for a third angle of incidence of the input laser beam.

<figref idref="DRAWINGS">FIG. 13</figref> illustrates an alternative embodiment of the laser amplifier of <figref idref="DRAWINGS">FIG. 1</figref> including side mirrors.

<figref idref="DRAWINGS">FIG. 14</figref> illustrates an alternative embodiment of the laser amplifier of <figref idref="DRAWINGS">FIG. 1</figref> showing undoped end sections in the laser active slab.

<figref idref="DRAWINGS">FIG. 15</figref> illustrates an alternative embodiment of the laser amplifier of <figref idref="DRAWINGS">FIG. 1</figref> including an angled entrance face and an exit facet in the laser active slab;

<figref idref="DRAWINGS">FIG. 16</figref> illustrates an alternative embodiment of the laser amplifier of <figref idref="DRAWINGS">FIG. 1</figref> including a one-dimensional array of optical fibers used for delivery of pump power to the laser active slab;

<figref idref="DRAWINGS">FIG. 17</figref> illustrates an alternative embodiment of the laser amplifier of <figref idref="DRAWINGS">FIG. 1</figref> pumped with a two-dimensional laser diode array having multiple diode bars;

<figref idref="DRAWINGS">FIG. 18</figref> illustrates an alternative embodiment of the laser amplifier of <figref idref="DRAWINGS">FIG. 17</figref>;

<figref idref="DRAWINGS">FIG. 19</figref> illustrates a setup for determining optimal wedge angle for a laser active slab, in accordance with the present invention;

<figref idref="DRAWINGS">FIG. 20</figref> is a flow diagram illustrating a procedure for determining optimal wedge angle for a laser active slab using the setup of <figref idref="DRAWINGS">FIG. 19</figref>;

<figref idref="DRAWINGS">FIGS. 21A and 21B</figref> are graphs of laser output power as a function of pump power as may be obtained using the setup of <figref idref="DRAWINGS">FIG. 19</figref>;

<figref idref="DRAWINGS">FIG. 22</figref> is a diagrammatical illustration of a laser amplifier that includes a single diode pump source;

<figref idref="DRAWINGS">FIG. 23</figref> is a diagrammatical illustration of a laser amplifier that includes a plurality of diode pump sources;

<figref idref="DRAWINGS">FIG. 24</figref> is a diagrammatical illustration of an embodiment of a two-pass laser amplifier, in accordance with the present invention;

<figref idref="DRAWINGS">FIG. 25</figref> is a diagrammatical illustration of an alternative embodiment of the two-pass laser amplifier of <figref idref="DRAWINGS">FIG. 24</figref>;

<figref idref="DRAWINGS">FIG. 26</figref> is a diagrammatical illustration of another alternative embodiment of the two-pass laser amplifier of <figref idref="DRAWINGS">FIG. 24</figref>;

<figref idref="DRAWINGS">FIG. 27</figref> is a diagrammatical illustration of yet another alternative embodiment of the two-pass laser amplifier of <figref idref="DRAWINGS">FIG. 24</figref>;

<figref idref="DRAWINGS">FIG. 28</figref> is a diagrammatical illustration of still another alternative embodiment of the two-pass laser amplifier of <figref idref="DRAWINGS">FIG. 24</figref>;

<figref idref="DRAWINGS">FIG. 29</figref> is a diagrammatical illustration of an embodiment of a three-pass laser amplifier, in accordance with the present invention;

<figref idref="DRAWINGS">FIG. 30</figref> is a diagrammatical illustration of an embodiment of a four-pass laser amplifier, in accordance with the present invention;

<figref idref="DRAWINGS">FIG. 31</figref> is a diagrammatical illustration of an embodiment of a laser amplifier including volume Bragg gratings, in accordance with the present invention;

<figref idref="DRAWINGS">FIG. 32</figref> is a diagrammatical illustration of an embodiment of a laser amplifier pumped through longitudinal faces, in accordance with the present invention;

<figref idref="DRAWINGS">FIG. 33</figref> is a diagrammatical illustration of an embodiment of a laser amplifier having convex lateral faces, in accordance with the present invention;

<figref idref="DRAWINGS">FIG. 34</figref> is a diagrammatical illustration of a setup for the configuration of a laser oscillator, in accordance with the present invention;

<figref idref="DRAWINGS">FIG. 35</figref> is a diagrammatical illustration of a ring laser oscillator, in accordance with the present invention;

<figref idref="DRAWINGS">FIG. 36</figref> is a diagrammatical illustration of a cascaded laser amplifier chain, in accordance with the present invention;

<figref idref="DRAWINGS">FIG. 37</figref> is a diagrammatical illustration of an amplified spontaneous emission source, in accordance with the present invention; and

<figref idref="DRAWINGS">FIG. 38</figref> is a diagrammatical illustration of a laser amplifier having a laser active slab with undoped lateral sections, in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

There is shown in <figref idref="DRAWINGS">FIGS. 1 and 2</figref> simplified diagrammatical illustrations of a laser amplifier <b>10</b> for an incoming laser beam <b>31</b> having a specified lasing wavelength, the laser amplifier <b>10</b> including a laser active slab <b>11</b>, a diode bar <b>21</b>, and an optional microlens <b>23</b>. The laser active slab <b>11</b> includes a first lateral face <b>13</b>, a second lateral face <b>15</b> opposed to the first lateral face <b>13</b>, a first longitudinal face <b>17</b>, and a second longitudinal face <b>19</b> opposed to the first longitudinal face <b>17</b>. The laser active slab <b>11</b> may comprise a block of laser active material such as: Nd:YAG, Nd:YLF, Nd:YVO₄, Nd:GdVO₄, Yb:YAG, Yb:YLF, Tm:YAG, Tm:YLF, and Tm:YAlO.

The diode bar <b>21</b> comprises a plurality of semiconductor diode lasers (not shown), operating at a pump power wavelength and emitting a respective plurality of radiation beams, as represented by diode emitter beamlets <b>25</b><i>a</i>-<b>25</b><i>e</i>. The laser active slab <b>11</b> may include an optical coating <b>27</b>, such as a thin-film dichroic coating, disposed on the first lateral face <b>13</b>, highly reflecting at the wavelength of the incoming laser beam <b>31</b>, and also highly transmitting at the pump power wavelength of the diode emitter beamlets <b>25</b><i>a</i>-<b>25</b><i>e</i>. In one embodiment, the size of the laser active slab <b>11</b> may be approximately twelve to twenty millimeters between the first longitudinal face <b>17</b> and the second longitudinal face <b>19</b>, and three to six millimeters between the first lateral face <b>13</b> and the second lateral face <b>15</b>.

The laser amplifier <b>10</b> functions to amplify the input laser beam <b>31</b> which is oriented at a specified angle of incidence and positioned at a specified distance so as to pass through an input window <b>33</b> and travel through a gain sheet <b>35</b> in the laser active slab <b>11</b>. The gain sheet <b>35</b> is produced by the diode emitter beamlets <b>25</b><i>a</i>-<b>25</b><i>e </i>as explained in greater detail below. The input laser beam <b>31</b> passes through the gain sheet <b>35</b> and emerges from the laser active slab <b>11</b> as an amplified laser beam <b>39</b>, via an output window <b>37</b>. The input window <b>33</b> and the output window <b>37</b> may each include a window anti-reflection coating <b>29</b> for the wavelength of the input laser beam <b>31</b>.

The laser amplifier <b>10</b> may further include a second diode bar <b>41</b> and a second microlens <b>43</b>, used in conjunction with the diode bar <b>21</b> and the microlens <b>23</b>, to produce the gain sheet <b>35</b>. The second diode bar <b>41</b> may comprise a plurality of semiconductor diode lasers (not shown), operating at the pump power wavelength and emitting a respective plurality of radiation beams, as represented by secondary diode emitter beamlets <b>45</b><i>a</i>-<b>47</b><i>e</i>. The laser active slab <b>11</b> may also include a second optical coating <b>47</b> highly reflecting at the wavelength of the input laser beam <b>31</b> and disposed on the second lateral face <b>15</b>. The second optical coating <b>47</b> may also be highly transmitting at the wavelength of the secondary diode emitter beamlets <b>45</b><i>a</i>-<b>47</b><i>e. </i>

As best seen in <figref idref="DRAWINGS">FIG. 2</figref>, the laser active slab <b>11</b> also includes a first transverse face <b>51</b> and an opposed second transverse face <b>53</b>, where the first transverse face <b>51</b> may be substantially parallel to the second transverse face <b>53</b>. In one embodiment, the size of the laser active slab <b>11</b> may be approximately two millimeters between the first transverse face <b>51</b> and the second transverse face <b>53</b>. The laser amplifier <b>10</b> may include an optional cooling surface <b>61</b>, such as a heat sink, maintained in thermal contact with the first transverse face <b>51</b> by means of an intervening conductive material <b>63</b>, such as indium foil, solder, thermally-conductive RTV, or thermally-conductive epoxy, for example. The laser amplifier <b>10</b> may also include a second optional cooling surface <b>65</b> attached to the second transverse face <b>53</b> by the conductive material <b>63</b>. The cooling surfaces <b>61</b> and <b>63</b> may comprise, for example, microchannel cooler blocks, finned heat sinks cooled by convection or forced air, or blocks of copper or other thermally-conductive material cooled by water or air. Alternatively, the first transverse face <b>51</b> and the second transverse face <b>53</b> may be directly cooled by a flow of water or air. For clarity of illustration, the cooling surface <b>61</b>, the conductive material <b>63</b>, and the second cooling surface <b>65</b> are not shown in <figref idref="DRAWINGS">FIG. 1</figref>, and the incoming laser beam <b>31</b> is not shown in <figref idref="DRAWINGS">FIG. 2</figref>.

As can be seen, the diode emitter beamlets <b>25</b><i>a</i>-<i>e </i>are incident approximately normal to the first lateral face <b>13</b> and the secondary diode emitter beamlets <b>45</b><i>a</i>-<b>37</b><i>e </i>are incident approximately normal to the second lateral face <b>15</b>. The diode bar <b>21</b> and the diode bar <b>41</b> may each comprise a one-dimensional laser array bar having an emitter separation pitch ranging from about 0.1 to one millimeter, such as available from Coherent Inc of Santa Clara, Calif. or Cutting Edge Optronics of St. Charles, Mo. The diode bars <b>21</b> and <b>41</b> may each produce at least twenty to eighty watts of pump power. When present, the microlens <b>23</b> may function to collimate the emissions of the diode bar <b>21</b> along the fast-axis direction such that the diode emitter beamlets <b>25</b><i>a</i>-<i>e </i>have a quasi-collimated divergence of approximately one to three degrees FWHM and a FWHM height of approximately 0.3 to 0.5 mm at the first lateral face <b>13</b>. With such a configuration, the gain sheet <b>35</b> may have a thickness, denoted as ‘t’ in the illustration, of approximately 0.3 to 0.5 mm. The diode emitter beamlets <b>25</b><i>a</i>-<i>e </i>may have an uncollimated divergence of approximately ten degrees FWHM along the slow-axis direction. The microlens <b>23</b> may comprise a lens such as manufactured by Doric Lenses of Quebec, Canada, LIMO of Dortmund, Germany, and Blue Sky Research of Milpitas, Calif.

The microlens <b>23</b> may be positioned from about one to thirty millimeters from the first lateral face <b>13</b> so as to provide for an overlapping of adjacent diode emitter beamlets <b>25</b><i>a</i>-<i>e </i>at the first lateral face <b>13</b>. Similarly, the microlens <b>43</b> may be positioned from about one to thirty millimeters from the second lateral face <b>15</b> wherein the diode emitter beamlets <b>45</b><i>a</i>-<i>e </i>have a quasi-collimated fast-axis divergence of approximately one to three degrees FWHM and an uncollimated slow-axis divergence of approximately ten degrees FWHM to provide for overlapping of adjacent diode emitter beamlets <b>45</b><i>a</i>-<i>e </i>at the second lateral face <b>15</b>.

In an alternative embodiment, shown in <figref idref="DRAWINGS">FIG. 3</figref>, the diode bar <b>21</b> may be oriented at an angle below the normal to the first lateral face <b>13</b>, here denoted as angle ‘A’ which may be approximately 1° to 5° as measured in the plane of the first longitudinal face <b>17</b>. The angle A can be determined as a function of the diameter of diode emitter beamlets <b>25</b><i>a</i>-<i>e </i>along the fast axis, the divergence of the diode emitter beamlets <b>25</b><i>a</i>-<i>e </i>along the fast axis, the refractive index and width of the medium of the laser active slab <b>11</b>, and the relative distances between the first lateral face <b>13</b>, the microlens <b>23</b>, and the diode bar <b>21</b>. By thus orienting the diode bar <b>21</b>, for example, any of the diode emitter beamlets <b>25</b><i>a</i>-<i>e </i>passing through the laser active slab <b>11</b> and out of the second lateral face <b>15</b> are directed away from possibly damaging the second diode bar <b>41</b>. Similarly, the second diode bar <b>41</b> may be oriented at the angle A below the normal to the second lateral face <b>15</b>, as shown, to avoid causing damage to the diode bar <b>21</b>. Alternatively, either or both of the diode bar <b>21</b> and the second diode bar <b>41</b> may be oriented at the angle A above (not shown), rather than below, the normals to the respective lateral faces <b>13</b> and <b>15</b>.

The input laser beam <b>31</b> is oriented to pass through the gain sheet <b>35</b> in the laser active slab <b>11</b> and is thus amplified, generally as shown in <figref idref="DRAWINGS">FIG. 4</figref>. For clarity of illustration, the second diode bar <b>41</b> and the second microlens <b>43</b> are not shown. The input laser beam <b>31</b> enters the laser active slab <b>11</b> at the input window <b>33</b>, is refracted at the input window <b>33</b>, and follows an internal zig-zag path <b>71</b>, comprising a plurality of path legs <b>71</b><i>a</i>-<b>71</b><i>h</i>, through the gain sheet <b>35</b> and bounded by the first lateral face <b>13</b> and the second lateral face <b>15</b>. It can be appreciated by one skilled in the relevant art that the direction of propagation may be in the reverse direction as well, that is, the input laser beam <b>31</b> can enter at the output window <b>37</b> and exit through the input window <b>33</b>.

The first lateral face <b>13</b> is spaced from the second lateral face <b>15</b> by a distance equal to the width of the laser active slab <b>11</b>, denoted as a dimension ‘W.’ Accordingly, each path leg <b>71</b><i>a</i>-<b>71</b><i>h </i>is approximately

<maths id="MATH-US-00001" num="00001"><math overflow="scroll"><mfrac><mi>W</mi><mrow><mi>cos</mi><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mi>B</mi></mrow></mfrac></math></maths><img file="US7961771B2_D0001.tif" /><br /> in length. Although only eight path legs <b>71</b><i>a</i>-<b>71</b><i>h </i>are shown in the illustration, the input laser beam <b>31</b> may make more or fewer passes through the laser active slab, as explained in greater detail below. The optical coatings <b>27</b> and <b>47</b> may also be highly-reflecting at the wavelength of the input laser beam <b>31</b>. This configuration provides for reflection of the internal propagating laser radiation at a smaller angle, here denoted by an angle ‘B’ at a path vertex <b>73</b>, than would be required if the optical coatings <b>27</b> and <b>47</b> were not present. Accordingly, in an alternative embodiment, the optical coatings <b>27</b> and <b>47</b> are not disposed on the respective lateral faces <b>13</b> and <b>15</b>, the input laser beam <b>31</b> may be input at a larger injection angle ‘F,’ and the internal zig-zag path <b>71</b> may propagate by means of total internal reflection (TIR) at the lateral faces <b>13</b> and <b>15</b>, as is well-understood in the relevant art.

By following the internal zig-zag path <b>71</b>, the input laser beam <b>31</b> makes multiple passes through the gain sheet <b>35</b> and is thereby amplified, as well-known in the relevant art, to emerge at the output window <b>37</b> as the amplified laser beam <b>39</b>. The input laser beam <b>31</b> may be a TEM₀₀beam, or a near-TEM₀₀quality beam. The thickness of the gain sheet <b>35</b> (see <figref idref="DRAWINGS">FIG. 2</figref>, above) is preferably approximately equal to or slightly greater than the transverse dimension of the input laser beam <b>31</b> (i.e., a 1/e²diameter) so as to achieve optimal mode matching, which provides for high gain and efficient extraction of power from the laser amplifier <b>10</b>.

<figref idref="DRAWINGS">FIGS. 5 and 6</figref>, which are not drawn to scale, provide diagrammatical illustrations of the geometric configuration of the laser active slab <b>11</b> which functions to mitigate parasitic oscillation or parasitic amplified spontaneous emission <b>75</b> that might otherwise be generated within the laser active slab <b>11</b>. For clarity of illustration, the optical coating <b>27</b>, the window anti-reflection coatings <b>29</b>, and the second optical coating <b>47</b> are not shown. The first lateral face <b>13</b> is substantially normal to the first longitudinal face <b>17</b> and to the second longitudinal face <b>19</b>. The second lateral face <b>15</b> is oriented to form a wedge lateral dihedral angle ‘C’ with the first lateral face <b>13</b>, where the wedge lateral dihedral angle C is at least 0.1°. It should be understood that the wedge lateral dihedral angle C has been exaggerated for clarity of illustration.

This configuration forms an asymmetrical trapezoid prism and thus provides for parasitic amplified spontaneous emission <b>75</b>, propagating through the gain sheet <b>35</b> in generally lateral directions, to ‘walk’ out of the laser active slab <b>11</b> through the second longitudinal face <b>19</b> by alternately reflecting from the first lateral face <b>13</b> and the second lateral face <b>15</b> while migrating to the second longitudinal face <b>19</b>, as generally represented by parasitic path legs <b>77</b><i>a</i>-<b>77</b><i>d </i>shown in <figref idref="DRAWINGS">FIG. 6</figref>. It should be understood that the illustration has been simplified and that the parasitic amplified spontaneous emission <b>75</b> may exhibit a greater number of reflections from the first lateral face <b>13</b> and the second lateral face <b>15</b> than shown. In general, that the number of such reflections is a function of the dimensions of the laser active slab <b>11</b> and of the wedge lateral dihedral angle C. It can be appreciated by one skilled in the relevant art that, because a wedged amplifier functions to minimize parasitic amplified spontaneous emission, twenty to eighty watts or more of pump power can be used to generate the gain sheet in the amplifier without producing the magnitude of thermomechanical stresses present in conventional diode-pumped active slabs.

In an alternative embodiment, shown in <figref idref="DRAWINGS">FIG. 7</figref>, a laser active slab <b>81</b> comprising a block of the laser active material as described above may be configured as a symmetrical trapezoid prism shown in <figref idref="DRAWINGS">FIG. 7</figref>, which is not drawn to scale. The laser active slab <b>81</b> includes a gain sheet <b>87</b>, and a first longitudinal face <b>91</b> which is shown as parallel, in the plane of illustration, to a second longitudinal face <b>93</b>. However, it should be understood that the first longitudinal face <b>91</b> can be angled or counter-wedged (not shown) with respect to the second longitudinal face <b>93</b> without impairing the amplified spontaneous emission suppression properties of the laser active slab <b>81</b>. A first lateral face <b>83</b> is oriented to form a wedge lateral dihedral angle ‘D’ with a second lateral face <b>85</b>, where the wedge lateral dihedral angle D is nominally 0.5° and is preferably in the range of about 0.1°≦D≦about 2.0°. The first lateral face <b>83</b> and the second lateral face <b>85</b> each form an acute dihedral angle with the first longitudinal face <b>91</b>.

The laser active slab <b>81</b> may include one or more additional features, shown in <figref idref="DRAWINGS">FIG. 8</figref>, to mitigate the propagation of amplified spontaneous emission or parasitic radiation through the gain sheet <b>87</b> in longitudinal directions. For example, the second longitudinal face <b>93</b> may be sloped so as to define an acute dihedral angle ‘E’ with a transverse face <b>89</b> as shown, where 85.0°≦E≦89.9°. Alternatively, the first longitudinal face <b>91</b> may be similarly sloped (not shown) so as to form an acute dihedral angle with the transverse face <b>89</b>, or the first longitudinal face <b>91</b> may be both sloped (not shown) and parallel, or anti-parallel, to the second longitudinal face <b>95</b>. Additionally, a longitudinal optical coating <b>95</b> may be disposed on either or both the first longitudinal face <b>91</b> and the second longitudinal face <b>93</b> (not shown) where the longitudinal optical coating <b>95</b> is anti-reflecting at the wavelength of a laser beam to be amplified. It can be appreciated by one skilled in the relevant art that the features shown in <figref idref="DRAWINGS">FIG. 8</figref> may be utilized in any other embodiments disclosed in the present specification and that such other configurations lie within the scope of the present invention.

The uniformity of diode-pumped gain along the width W of the laser active slab <b>11</b>, or the laser active slab <b>81</b>, is generally determined by the amount of pump power absorbed in the laser active slab <b>11</b> or <b>81</b>. In the laser active slab <b>11</b>, for example, the uniformity of diode-pumped gain is thus a function of the diode pump power which is incident on the first lateral face <b>13</b> and transmitted laterally through the second lateral face <b>15</b>, that is, the fraction of the incident pump power which is not absorbed in the gain sheet <b>35</b>. As is well known to someone of ordinary skill in the art, the amount of pump power absorbed in the laser active slab <b>11</b> depends on numerous factors, such as: the doping concentration of laser active ions in the active material, the overall path length for absorption of diode pump power, the peak diode pump wavelength, the spectral profile of the diode pump power, and the power density and polarization state of the diode pump power.

Referring to graph <b>101</b> in <figref idref="DRAWINGS">FIG. 9</figref>, a set of curves <b>103</b>-<b>109</b> illustrates how diode-pumped gain at the wavelength of the input laser beam <b>31</b> varies along the width W of the laser active slab <b>11</b>, where the laser active slab <b>11</b> is pumped both at the first lateral face <b>13</b> and the second lateral face <b>15</b> with respective diode bars <b>21</b> and <b>41</b>. Derivation of the curves <b>103</b>-<b>109</b> assumes that diode pump light is absorbed in the laser active slab <b>11</b> in accordance with the well known Beer's-law exponential profile, and that laser gain at a particular point along the width of the laser active slab <b>11</b> is linearly proportional to the diode pump power level at the particular point in the laser active slab <b>11</b>. The graph <b>101</b> illustrates how gain varies along the slab width for different levels of pump power transmitted through the slab: 5%, 10%, 15%, and 20%. In certain cases, such as when a quasi-three-level laser active material is being pumped by the diode bars <b>21</b> and <b>41</b>, it may be desirable to provide a higher level of pump power transmission through the laser active slab <b>11</b>, so that the gain profile is more uniform along the width of the laser active slab <b>11</b>. Such a configuration results in greater net gain through the laser amplifier <b>10</b>, but there may be a tradeoff in that the efficiency of the laser amplifier <b>10</b> is reduced.

The magnitude of the unsaturated gain-per-unit-length coefficient, or gain coefficient, g_O, established in the diode-pumped gain sheet <b>35</b> can be determined from the pump power density as a function of the total diode pump power and the thickness “t” of the gain sheet <b>35</b> (see <figref idref="DRAWINGS">FIG. 2</figref>). Accordingly, g_Ocan be approximately doubled by doubling the pump power incident on the laser active slab <b>11</b>, or by halving the thickness of the gain sheet <b>35</b>, assuming there are no gain-reducing processes dependent on pump power density occurring in the laser active slab <b>11</b>.

The unsaturated power/intensity gain-per-pass, G_o, may be given by G_o=exp(g_OL), where g_Ois the unsaturated gain coefficient, and L is the total length of one zig-zag pass through the laser active slab <b>11</b>. In the embodiment of <figref idref="DRAWINGS">FIG. 4</figref>, for example, the single-pass gain length L is the sum of the lengths of the path legs <b>71</b><i>b</i>-<b>71</b><i>g </i>and the portions of the first and last legs, <b>71</b><i>a </i>and <b>71</b><i>h</i>, which lie in the gain sheet <b>35</b>. The single-pass gain length, L, is thus determined by the width W of the laser active slab <b>11</b> and the number of path legs that overlap with the diode-pumped gain sheet <b>35</b>. As can be appreciated by one skilled in the relevant art, the number of path legs which lie in the gain sheet <b>35</b> can be determined as a function of the injection angle ‘F’ of the input laser beam <b>31</b> into the slab, the refractive index of the slab material at the lasing wavelength, and the longitudinal length of the gain sheet <b>35</b>.

As shown in <figref idref="DRAWINGS">FIG. 10</figref>, an input laser beam <b>113</b>, or resonator mode, being amplified is injected into a laser active slab <b>111</b> through an input window <b>115</b>, follows a zig-zag path <b>131</b>, and exits through an output window <b>117</b>, each of which windows may include an optical coating <b>119</b> highly transmitting at the wavelength of the input laser beam <b>113</b>. The input laser beam <b>113</b> is at an angle of incidence denoted as angle ‘F’ from the normal to a first lateral face <b>121</b>, and the zig-zag path <b>131</b> is preferably in the plane defined by a gain sheet <b>129</b> in the laser active slab <b>111</b>. The gain sheet <b>129</b> may be formed by diode bars (not shown) adjacent the lateral face <b>121</b> and a second lateral face <b>123</b> as above in <figref idref="DRAWINGS">FIGS. 1 and 4</figref>, for example. The angle of incidence F provides for the input laser beam <b>113</b> to undergo multiple internal reflections alternating between the lateral faces <b>121</b> and <b>123</b>, as represented by end points of path legs <b>131</b><i>a</i>-<b>131</b><i>f</i>, while making at least one zig-zag pass through the gain sheet <b>129</b>, approximately as shown. The first lateral face <b>121</b> and the second lateral face <b>123</b> may each include an optical coating <b>125</b> highly reflecting at the wavelength of the input laser beam <b>113</b> and highly transmitting at the wavelength of the pump power producing the gain sheet <b>129</b>.

The first lateral face <b>121</b> is further oriented to form the wedge lateral dihedral angle C with the second lateral face <b>123</b>, where the laser active slab <b>111</b> has slightly wider lateral dimension at the output window <b>117</b> than at the input window <b>115</b>. The points of multiple internal reflections, and the path legs <b>131</b><i>a</i>-<i>f</i>, thus define a series of path vertices <b>133</b><i>a</i>-<b>133</b><i>c </i>that are non-uniformly spaced along the second lateral face <b>125</b>. Accordingly, as can be appreciated by one skilled in the relevant art, the fold angle formed by the path legs <b>131</b><i>e </i>and <b>131</b><i>f </i>is necessarily larger than the fold angle formed by the path legs <b>131</b><i>c </i>and <b>131</b><i>d </i>by an amount equal to twice the lateral wedge angle C, and the fold angle formed by the path legs <b>131</b><i>c </i>and <b>131</b><i>d </i>is likewise larger than the fold angle formed by the path legs <b>131</b><i>a </i>and <b>131</b><i>b </i>by the amount equal to twice the lateral wedge angle C. That is, the fold angles vary as a function of corresponding vertex spacings or positions along the lateral faces <b>121</b> and <b>123</b>. Similarly, the length of the path leg <b>131</b><i>b </i>is greater than the length of the path leg <b>131</b><i>a</i>, and each downstream path leg is greater than a previous path leg, when the input laser beam <b>113</b> has a direction of propagation from the input window <b>115</b> to the output window <b>117</b>, that is, from the narrower to the wider end of the laser active slab <b>111</b>.

Preferably, diode bars (not shown) adjacent the laser active slab <b>111</b> are positioned such that the diode emitter beamlets (not shown) produced by the first diode bar overlap in the slow-axis direction at the first lateral face <b>121</b>, and the diode emitter beamlets (not shown) produced by the second diode bar overlap in the slow axis direction at the second lateral face <b>123</b>, so that the resultant gain sheet <b>129</b> is substantially uniform. Accordingly, the path legs <b>131</b><i>a</i>-<b>131</b><i>f </i>remain largely within the gain sheet <b>129</b> even though the path vertices <b>133</b><i>a</i>-<i>c </i>are non-uniformly spaced along the second lateral face <b>123</b>. The distance between the vertices <b>133</b><i>b </i>and <b>133</b><i>c </i>is larger than the distance between the vertices <b>133</b><i>a </i>and <b>133</b><i>b</i>. That is, the disclosed configuration functions as described above even with standard diode bars having evenly-spaced diode emitters as it is not necessary to match up path vertices <b>133</b><i>a</i>-<i>c</i>, for example, with the positions of the adjacent diode emitters (not shown) because the gain sheet <b>129</b> is substantially uniform. The disclosed configurations may somewhat reduce the laser gain that can be achieved in the laser active slab <b>111</b>, in comparison to prior art configurations having uniformly-spaced path vertices and diode emitter beamlets precisely matched to the vertex positions of the zig-zag laser beam path. However, the disclosed configurations described herein provide for greatly simplified alignment requirements to achieve high gain, in comparison to conventional configurations which may include high-power laser diode bars having emitter-to-emitter spacings significantly smaller than one millimeter.

The single-pass gain length ‘L’ of the zig-zag path <b>131</b> is a function of angle of incidence F, among other parameters. This is best illustrated by comparison of the six-leg zig-zag path <b>131</b> of <figref idref="DRAWINGS">FIG. 10</figref> with an eight-leg zig-zag path <b>135</b> in <figref idref="DRAWINGS">FIG. 11</figref> and an eleven-leg zig-zag path <b>137</b> in <figref idref="DRAWINGS">FIG. 12</figref>. For purpose of comparison, the laser active slab <b>111</b>, which comprises a block or slab of the laser active material as described above, can be specified to have a refractive index of about 2.17, a length of approximately fifteen millimeters, and a width of approximately six millimeters. The wedge lateral dihedral angle C may be about 1.0 degree and the gain sheet <b>129</b> may have a longitudinal dimension of about ten millimeters. The angle of incidence F in <figref idref="DRAWINGS">FIG. 10</figref> is about 32 degrees to give a value for the length of the zig-zag path <b>131</b> in the gain sheet <b>129</b> (i.e., the single-pass gain length L), of at least 33 millimeters (i.e., L≈5.5×6 mm).

The angle of incidence F in <figref idref="DRAWINGS">FIG. 11</figref> is about 21 degrees to give a value for the length of the zig-zag path <b>135</b> in the gain sheet <b>129</b> of at least 42 millimeters (i.e., L≈7.0×6 mm). The angle of incidence F in <figref idref="DRAWINGS">FIG. 12</figref> is about 7.5 degrees to give a value for the single-pass gain length L of at least 57 millimeters (i.e., L≈9.5×6 mm). In this configuration, the input laser beam <b>113</b> follows the zig-zag path <b>137</b> which exits from the laser active slab <b>111</b> at an alternate output window <b>139</b>, where the output window <b>139</b> may include the optical coating <b>119</b>. For an unsaturated gain coefficient, g_Oof about 1.5 cm⁻¹, a value which can be achieved in an active medium such as Nd:YVO₄pumped with two twenty-watt diode bars, for example, the unsaturated gain G_ovalues corresponding to the single-pass gain lengths L of <figref idref="DRAWINGS">FIGS. 10-12</figref> are about 141, 544, and 5,166, respectively.

The process of achieving high gain and efficient extraction of power and energy from the diode-pumped laser amplifier <b>10</b> in <figref idref="DRAWINGS">FIG. 4</figref>, for example, includes mode matching the input laser beam <b>31</b> to the gain sheet <b>35</b>. Mode matching can be accomplished by spatially overlapping the volume occupied by the amplified beam following the zig-zag path <b>71</b> with the diode-pumped volume defined by the gain sheet <b>35</b>. In general, proper mode matching in the transverse direction of the laser active slab <b>11</b> requires that the thickness of the gain sheet <b>35</b> be nominally equal to or slightly larger than the vertical diameter of the input laser beam <b>31</b>, or mode, at essentially all points along the zig-zag path <b>71</b>. Alternatively, the input laser beam <b>31</b> may be focused in a vertical plane perpendicular to the first transverse face <b>51</b>, such that at one or more points along the zig-zag path <b>71</b>, the transverse diameter of the input laser beam <b>31</b>, or mode, may be substantially smaller than the thickness of the gain sheet <b>35</b>.

Proper mode matching in the horizontal plane of the zig-zag path <b>71</b> may depend upon the application in which the laser amplifier <b>10</b> is being used, for example, as a pre-amplifier, as a power amplifier, or in a laser oscillator. In general, once the thickness of the gain sheet <b>35</b> and the unsaturated gain-per unit-length (g_O) have been determined, the input angle F and the corresponding number of vertices in the zig-zag path <b>71</b> are adjusted to achieve desired single-pass gain length L and extraction efficiency consistent with the particular geometrical configuration of the laser active slab <b>11</b>.

The process of achieving high gain and efficient extraction of power and energy from the diode-pumped laser amplifier <b>10</b>, for example, may include increasing the diameter of the input laser beam <b>31</b>, or mode, in a transverse plane using beam-shaping methods well known to one of ordinary skill in the relevant art. Preferably, the transverse beam diameter inside the laser active slab <b>11</b> is maximized along the zig-zag path <b>71</b> without incurring significant beam clipping and resultant diffraction losses either at the edges of the second optical coating <b>47</b> or at the edges of the laser active slab <b>11</b>, as the input laser beam <b>31</b> enters and exits the laser active slab <b>11</b> through the input window <b>33</b> and the output window <b>37</b>, respectively.

When the laser amplifier <b>10</b> is operated at power levels of more than a few watts of average diode pump power, the cooling surfaces <b>61</b> and <b>65</b>, shown in <figref idref="DRAWINGS">FIG. 2</figref>, may be used to remove heat from the laser active slab <b>11</b>. Cooling the first transverse face <b>51</b> and the second transverse face <b>53</b> in this manner produces a substantial thermal gradient in the transverse direction, that is, along a path normal to both the first transverse face <b>51</b> and the second transverse face <b>53</b>. Such a linear thermal gradient permits the amplification of a linearly polarized laser beam, whether the polarization is in the transverse direction or in the plane of the gain sheet <b>35</b>, without significant distortion of the polarization state of the laser beam or mode being amplified.

Because the thermal gradient is perpendicular to the plane of zig-zag path <b>71</b> (i.e., the plane of the gain sheet <b>35</b>) when the laser active slab <b>11</b> is cooled by means of the cooling surfaces <b>61</b> and <b>65</b> at the first and second transverse faces <b>51</b> and <b>53</b>, the thermal gradient may result in an undesirable thermally-induced lensing and an optical wedge that acts to steer the input laser beam <b>31</b>, or mode, in a direction substantially perpendicular to the plane of the zig-zag path <b>71</b>. This optical wedge effect is produced when the temperature differential between the first transverse face <b>51</b> and the second transverse face <b>53</b> results in an unacceptable level of beam steering. This temperature differential is a function of the diode pump power being absorbed in the laser active slab <b>11</b>, the thermal properties of the laser active material, differences in thermal resistances for top and bottom heat removal paths, and other factors well-know to one of ordinary skill in the relevant art. Accordingly, such thermally-induced optical wedge effects in the plane perpendicular to the plane of the zig-zag path <b>71</b> can be minimized by maintaining the cooling surfaces <b>61</b> and <b>65</b> at approximately the same temperature, and by providing top and bottom heat removal paths of the same or similar thermal resistances.

There is shown in <figref idref="DRAWINGS">FIG. 13</figref> an alternative laser amplifier <b>150</b> including a laser active slab <b>151</b>, first and second side mirrors <b>153</b> and <b>155</b>, first and second diode bars <b>157</b> and <b>159</b>, and first and second microlenses <b>161</b> and <b>163</b>. The first and second diode bars <b>157</b> and <b>159</b> cooperatively function to generate a gain sheet <b>175</b> in the laser active slab <b>151</b>. The side mirrors <b>153</b> and <b>155</b> include an optical coating <b>165</b> highly reflecting to the wavelength of the input laser beam <b>31</b> and highly transmitting at the wavelength of the pump power produced by the diode bars <b>157</b> and <b>159</b>. The side mirrors <b>153</b> and <b>155</b> are preferably positioned within two millimeters of lateral faces <b>167</b> and <b>169</b>, respectively. The lateral faces <b>167</b> and <b>169</b> each have an anti-reflection coating <b>171</b> designed for low reflectivity at both the lasing wavelength and the diode pump wavelength. The laser active slab <b>151</b> may comprise a block of the laser active material as described above.

Preferably, the anti-reflection coating <b>171</b> is applied along the entire lengths of the respective lateral faces <b>167</b> and <b>169</b>. The substrates of the side mirrors <b>153</b> and <b>155</b> are selected so as not to be absorbing at the wavelength of the diode pump power, and further may have an anti-reflection coating (not shown) at the diode pump wavelength on the surfaces facing the diode bars <b>157</b> and <b>159</b>. The first side mirror <b>153</b> forms a mirror dihedral angle (not shown for clarity of illustration) with the second side mirror <b>155</b>, where the mirror dihedral angle is equivalent to the wedge lateral dihedral angle C of <figref idref="DRAWINGS">FIG. 5</figref>, above. The side mirrors <b>153</b> and <b>155</b> are further aligned and mounted so as to produce the zig-zag path <b>173</b> through the gain sheet <b>175</b> in the laser active slab <b>151</b>.

There is shown in <figref idref="DRAWINGS">FIG. 14</figref> an embodiment of the present invention suitable for use with quasi-three-level laser materials, for example, where a laser amplifier <b>180</b> includes a laser active slab <b>181</b> and diode pump sources <b>191</b> and <b>193</b>. The particular configuration of laser active slab <b>181</b> functions to reduce unpumped regions in the laser active slab <b>181</b> that may otherwise act as absorbing regions at the lasing wavelength. The laser active slab <b>181</b> includes end sections <b>183</b> and <b>185</b> that are fabricated from the same or similar host material comprising an active section <b>187</b> of the laser active slab <b>181</b>. However, the end sections <b>183</b> and <b>185</b> are not doped with laser active ions, whereas the active section <b>187</b>, which includes a gain sheet <b>189</b>, is doped with laser active ions, such as in the laser active material as described above.

Pump power is provided to the laser active slab <b>181</b> by the diode pump sources <b>191</b> and <b>193</b>. The physical length of each of the diode pump sources <b>191</b> and <b>193</b> is approximately equal to the longitudinal length of the active section <b>187</b> so that the entire length of the active section <b>187</b> can be pumped with diode pump power. Because no active ions are present in the end sections <b>183</b> and <b>185</b>, there is no unwanted absorption loss at the lasing wavelength in the end sections <b>183</b> and <b>185</b>. The end sections <b>183</b> and <b>185</b> can be optically contacted and bonded to the active section <b>187</b> using diffusion bonding techniques, or other bonding methods not requiring epoxy, as have been made available through commercial suppliers such as VLOC Corporation (New Port Richey, Fla.) and Onyx Optics (Dublin, Calif.).

<figref idref="DRAWINGS">FIG. 15</figref> illustrates an alternative embodiment of a laser active slab <b>201</b> that includes a first lateral face <b>203</b> and a second lateral face <b>205</b>. The first lateral face <b>203</b> includes an angled entrance window <b>207</b> and an angled exit window <b>209</b>. The angled entrance window <b>207</b> and the angled exit window <b>209</b> are each oriented at an angle ‘G’ from the plane of the first lateral face <b>203</b>, where the angle G is about 3° to 5°. That is, the angled entrance window <b>207</b> forms a window dihedral angle of 175° to 177° with the first lateral face <b>203</b>. The angled entrance window <b>207</b> allows the input laser beam <b>31</b> to pass into the laser active slab <b>201</b> at a larger entrance angle from the normal to the first lateral face <b>203</b>, than would be the case if the angled entrance window <b>207</b> were coplanar with the first lateral face <b>203</b>. Accordingly, larger entrance and exit angles can help prevent partial blockage of the input laser beam <b>31</b> by, for example, a diode pump source (not shown) positioned close to the first lateral face <b>203</b>.

The illustration in <figref idref="DRAWINGS">FIG. 16</figref> shows a one-dimensional array <b>210</b> of optical fibers <b>211</b>-<b>219</b> used to deliver diode pump light from a diode bar (not shown) to a lateral face <b>223</b> of a laser active slab <b>221</b>. The lateral face <b>223</b> may include an optical coating <b>225</b> highly transmitting at a wavelength of the diode pump power and highly reflecting at the lasing wavelength. The individual optical fibers <b>211</b>-<b>219</b> are configured into the one-dimensional array <b>210</b> such that pump light is directed onto the lateral face <b>223</b> in a similar pattern as an adjacent diode bar, such as in the laser amplifier configurations described above. A collimator lens <b>227</b> may be used to quasi-collimate optical fiber emission in a transverse plane before entering the laser active slab <b>221</b>.

<figref idref="DRAWINGS">FIG. 17</figref> illustrates an alternative configuration for a laser amplifier <b>230</b>. A laser active slab <b>231</b> is pumped with a two-dimensional laser diode array <b>233</b> having multiple diode bars, <b>235</b><i>a</i>-<i>f</i>, such as available from Coherent Inc (Santa Clara, Calif.) or from Cutting Edge Optronics (St. Charles, Mo.). A corresponding set of microlenses <b>237</b><i>a</i>-<i>f </i>may be used to collimate the fast axis of each diode bar <b>235</b><i>a</i>-<i>f </i>in the two-dimensional laser diode array <b>233</b>, as described above for one-dimensional diode array bars. A set of multiple diode bar beams <b>239</b><i>a</i>-<i>f </i>propagate along generally parallel paths in a plane perpendicular to the diode junction planes. A first cylinder lens <b>241</b> may focus the multiple diode bar beams <b>239</b><i>a</i>-<i>f </i>to a common line focus <b>243</b>. A second cylinder lens <b>245</b> may re-collimate the diode bar beams <b>239</b><i>a</i>-<i>f </i>so that they again propagate parallel to one another as a set of de-magnified beams <b>247</b><i>a</i>-<i>f</i>, but with a greatly reduced pitch between the de-magnified beams <b>247</b><i>a</i>-<i>f </i>than in the original two-dimensional laser diode array <b>233</b>.

The de-magnified beams <b>247</b><i>a</i>-<i>f </i>enter the laser active slab <b>231</b> through a lateral face <b>251</b> which may include an optical coating <b>253</b> highly transmitting at wavelength of the pump power and highly reflecting at a lasing wavelength. Alternatively, a second two-dimensional laser diode array (not shown) could be added to pump the laser active slab <b>231</b> at a second lateral face <b>255</b>. The configuration of <figref idref="DRAWINGS">FIG. 17</figref> can be modified as shown in <figref idref="DRAWINGS">FIG. 18</figref> by eliminating the second cylinder lens <b>245</b> to form a laser amplifier <b>250</b>. The two-dimensional laser diode array <b>233</b> and the first cylinder lens <b>241</b> are positioned such that the line focus <b>243</b> is positioned just outside the laser active slab <b>231</b> adjacent the lateral face <b>251</b>, or on the lateral face <b>251</b> (not shown), or inside the laser active slab <b>231</b> adjacent the lateral face <b>251</b> (not shown). In the laser amplifier <b>250</b>, the angle at which the diode bar beams <b>239</b><i>a</i>-<i>f </i>converge or diverge after entering the laser active slab <b>231</b> is reduced, relative to the angles outside the laser active slab <b>231</b>, by the relevant refractive index ‘n’ of the slab material at the diode pump wavelength and pump polarization.

An optimal wedge lateral dihedral angle C (shown in <figref idref="DRAWINGS">FIG. 5</figref>), or an optimal wedge lateral dihedral angle D (shown in <figref idref="DRAWINGS">FIG. 7</figref>), may be found by empirically determining the smallest dihedral angle for a new, functional laser active slab (not shown) that minimizes the longest possible path length of parasitic amplified spontaneous emission. It can be appreciated that if the new laser active slab is designed to overcome emissions propagating along the longest folded path in the new laser active slab, the design of the new laser active slab will also overcome emissions propagating along shorter paths. Parasitic amplified spontaneous emission generated near a longitudinal face of a laser active slab typically follows a longer path length through the gain sheet than amplified spontaneous emission generated away from the longitudinal faces of the laser active slab. Parasitic amplified spontaneous emission is herein defined as amplified spontaneous emission that builds up in the gain sheet along a folded path different from the intended beam path for the input laser beam.

The empirical process for determining an optimal wedge lateral dihedral angle may use a simulation laser amplifier <b>260</b> such as shown in <figref idref="DRAWINGS">FIG. 19</figref> and may comprise a method described by reference to a flow diagram <b>300</b>, in <figref idref="DRAWINGS">FIG. 20</figref>. Referring to <figref idref="DRAWINGS">FIG. 19</figref>, the simulation laser amplifier <b>260</b> includes a simulator slab <b>261</b> comprising the same laser active material and having substantially the same dimensions to be used for the new laser active slab. The simulator slab <b>261</b> includes parallel lateral faces <b>263</b> and <b>265</b>, and wedged longitudinal faces <b>267</b> and <b>269</b>. That is, the simulator slab <b>261</b> is not wedged laterally, but the new laser active slab will be wedged at the optimal wedge lateral dihedral angle found using the method of <figref idref="DRAWINGS">FIG. 20</figref>. A gain sheet <b>271</b> may be produced in the simulator slab <b>261</b>, for example, by a first diode bar <b>273</b> with a first microlens <b>275</b> in combination with a second diode bar <b>277</b> and a second microlens <b>279</b>.

The design considerations used in determining the linear dimensions of the new laser slab include initially ascertaining the pump power and output power desired for a new laser amplifier. High pump and output power increase the probability of fracture in the new laser active slab, especially if the new laser active slab has a relatively large thickness-to-width ratio. In the disclosed embodiments, a relatively higher pump beam power intensity is directed into a relatively thin laser active slab to produce a gain sheet approximately one half (0.5) to one (1.0) millimeter thick.

A first simulator mirror <b>281</b> is disposed adjacent the simulator slab <b>261</b> between the lateral face <b>263</b> and the diode bar <b>273</b>. A second simulator mirror <b>283</b> is disposed adjacent the simulator slab <b>261</b> between the lateral face <b>265</b> and the diode bar <b>277</b>. The first simulator mirror <b>281</b> is parallel to the lateral face <b>263</b> and the second simulator mirror <b>283</b> is oriented to form a dihedral angle with the lateral face <b>265</b> and a mirror dihedral angle with the first simulator mirror <b>281</b>, as indicated by arrow <b>289</b>. This setup may be used to determine the optimal wedge lateral dihedral angle C for a particular active material, for the pump power density, and for other design parameters. Alternatively, to determine the optimal wedge lateral dihedral angle D, both the first simulator mirror <b>281</b> and the second simulator mirror <b>283</b> can be oriented at the same dihedral angle (not shown) to the respective lateral faces <b>263</b> and <b>265</b>. It can be appreciated by one skilled in the relevant art that the simulation laser amplifier <b>260</b> can be used to replicate any of the embodiments described above while allowing the wedge lateral dihedral angle C or D to be varied for the purpose of obtaining empirical data.

The lateral face <b>263</b> may have an optical coating <b>291</b> highly transmitting at the wavelength of the first diode bar <b>273</b> and highly transmitting at the wavelength of an input laser beam <b>290</b>. Similarly, the lateral face <b>265</b> may have an optical coating <b>293</b> highly transmitting at the wavelength of the second diode bar <b>277</b> and highly transmitting at the wavelength of the input laser beam <b>290</b>. The first simulator mirror <b>281</b> may include an optical coating <b>295</b> highly transmitting at the wavelength of the first diode bar <b>273</b> and highly reflecting at the wavelength of the input laser beam <b>290</b>, and the second simulator mirror <b>283</b> may include an optical coating <b>297</b> highly transmitting at the wavelength of the second diode bar <b>277</b> and highly reflecting at the wavelength of the input laser beam <b>290</b>. Operating parameters for the simulation laser amplifier <b>260</b> can be monitored by electronic equipment (not shown) that may measure and record the power output of the first diode bar <b>273</b>, the second diode bar <b>275</b>, and an amplified laser beam <b>299</b>.

Referring now to step <b>301</b> in the flow diagram <b>300</b> shown in <figref idref="DRAWINGS">FIG. 20</figref>, an initial mirror dihedral angle, indicated by the arrow <b>289</b> in <figref idref="DRAWINGS">FIG. 19</figref>, is selected to produce a zig-zag path for an incoming laser beam <b>290</b> entering the simulator slab <b>261</b> and exiting as an amplified laser beam <b>299</b>. The gain sheet <b>271</b> is generated in the simulator slab <b>261</b>, at step <b>303</b>, where the simulator slab <b>261</b> preferably includes the optical coatings <b>291</b> and <b>293</b>. The input laser beam <b>290</b> is directed into the gain sheet <b>271</b> and the pump power of the diode bars <b>273</b> and <b>277</b> is varied, at step <b>305</b>, from which action an amplified laser power vs. pump power curve, or input/output power curve, may be obtained for the initial mirror dihedral angle, at step <b>307</b>.

If, at decision block <b>309</b>, the input/output power curve obtained resembles a nonlinear power curve, such as curve <b>325</b> shown in <figref idref="DRAWINGS">FIG. 21A</figref> which falls off from a linear trace <b>327</b>, this may indicate the presence of unwanted parasitic amplified spontaneous emission in the gain sheet <b>271</b>. Accordingly, the mirror dihedral angle may be increased, at step <b>313</b>, and a new input/output power curve may be obtained, at step <b>307</b>. If, at decision block <b>309</b>, the input/output power curve obtained resembles a linear power curve <b>329</b>, such as shown in <figref idref="DRAWINGS">FIG. 21B</figref>, this may be an indication that the mirror dihedral angle is sufficiently large to prevent the buildup of unwanted parasitic amplified spontaneous emission in the gain sheet <b>271</b>. As a further check on the presence or absence of unwanted parasitic amplified spontaneous emission, at decision block <b>311</b>, the input laser beam <b>290</b> may be removed or blocked from the simulator slab <b>261</b> to determine whether a parasitic amplified spontaneous emission signal emitted from the gain sheet <b>271</b> exceeds a predetermined level, even with no input laser beam <b>290</b> present. If a parasitic amplified spontaneous emission signal exceeds the predetermined level, the mirror dihedral angle may be increased, at step <b>313</b>.

If there is no parasitic amplified spontaneous emission signal detected or if the parasitic amplified spontaneous emission signal is at or below the predetermined level, at decision block <b>311</b>, the current mirror dihedral angle may be used as the minimum functional wedge lateral dihedral angle for the new laser active slab, at step <b>315</b>. Alternatively, if no parasitic amplified spontaneous emission signal is detected, or if the parasitic amplified spontaneous emission signal is at or below the predetermined level, at decision block <b>311</b>, the mirror dihedral angle may be decreased, at step <b>317</b>, to verify that the current mirror dihedral angle is the optimal wedge lateral dihedral angle for the simulator active slab <b>261</b>. A new input/output power curve is obtained and evaluated, at step <b>317</b>. If the new input/output power curve obtained resembles a linear power curve, at decision block <b>319</b>, the mirror dihedral angle may be further decreased and yet another new input/output power curve obtained, at step <b>317</b>.

If the current input/output power curve obtained resembles a nonlinear power curve, at decision block <b>319</b>, a parasitic amplified spontaneous emission signal detection may be conducted to verify that the previous mirror dihedral angle qualifies as the optimal wedge lateral dihedral angle, at decision block <b>321</b>. If no parasitic amplified spontaneous emission signal is detected, or if the parasitic amplified spontaneous emission signal is at or below the predetermined level, at decision block <b>321</b>, the mirror dihedral angle may be decreased, at step <b>317</b>, and another new input/output power curve obtained and evaluated. If the parasitic amplified spontaneous emission signal exceeds the predetermined level, at decision block <b>321</b>, the previous mirror dihedral angle may be used as the optimal wedge lateral dihedral angle for the new laser active slab, at step <b>323</b>.

In an alternative embodiment, shown in <figref idref="DRAWINGS">FIG. 22</figref>, a laser amplifier <b>330</b> may include a single diode pump source <b>341</b> comprising a block of the laser active material as described above, a pump bar <b>343</b>, and a microlens <b>345</b>. The pump bar <b>343</b> may comprise an array of discrete singe-emitter diodes lasers, a one-dimensional laser diode array bar, a fiber-coupled one-dimensional laser diode array bar, or a two-dimensional laser diode array, for example, and may be continuous-wave or pulsed diode pump sources. The diode pump source <b>341</b> produces a gain sheet <b>333</b> in a wedged laser active slab <b>331</b> via a one-pass or a two-pass pumping operation.

For two-pass pumping, the wedged laser active slab <b>331</b> may include a lateral face <b>337</b> with a thin-film coating <b>347</b> that is highly reflecting at the lasing wavelength and highly transmitting at the diode pump wavelength, and a lateral face <b>339</b> with a thin-film coating <b>349</b> that is highly reflecting at the lasing wavelength and highly reflecting at the diode pump wavelength. Diode pump power not absorbed in the first pass through the wedged laser active slab <b>331</b> is reflected back toward the pump bar <b>343</b> for a second pass leg through the wedged laser active slab <b>331</b>. In this way, the path length for absorption of pump light is increased by a factor of two.

In another alternative embodiment, shown in <figref idref="DRAWINGS">FIG. 23</figref>, a laser amplifier <b>350</b> may include a linear array of diode pump sources <b>351</b>-<b>359</b>, such as semiconductor diode lasers, on a first side of a wedged laser active slab <b>361</b>, and a single diode pump sources <b>363</b> on a second side of the laser active slab <b>361</b>. The diode pump sources <b>351</b>-<b>59</b> and <b>343</b> may comprise an array of discrete single-emitter diode lasers, a one-dimensional laser diode array bar, a fiber-coupled one-dimensional laser diode array bar, or a two-dimensional laser diode array, for example, and may be continuous-wave or pulsed-diode pump sources. Other embodiments (not shown) may include diode pump sources all on the first side of the wedged laser active slab <b>361</b>. It should be understood that the pump sources need not be arranged symmetrically about the wedged laser active slab <b>361</b>, and that various types of pump sources can be used in the laser amplifier <b>350</b> need not all be similar to each other.

A wedged laser active slab in accordance with the present invention can be advantageously utilized in configurations that allow the incoming laser beam or resonator mode being amplified to make, for example, two, three, four, or more multi-passes through the diode-pumped gain sheet, as can be understood with additional embodiments described below. Geometrical methods alone can be utilized to separate input and output beams, or a Faraday rotator or isolator device can be included for beam separation. In preferred embodiments, the longitudinal faces of the laser active slab are wedged as well as the lateral faces to provide for dissipation of unwanted amplified spontaneous emission. There is shown in <figref idref="DRAWINGS">FIG. 24</figref> a diagram of a two-pass laser amplifier <b>370</b> including a wedged laser active slab <b>371</b> and one or more diode pump sources (not shown) used to produce a gain sheet <b>373</b>. An incoming laser beam <b>375</b> enters the wedged laser active slab <b>371</b> at a first window <b>377</b> and exits at a second window <b>379</b> after making a first zig-zag pass <b>381</b> through the gain sheet <b>373</b> in the wedged laser active slab <b>371</b>.

An external mirror <b>383</b> that is highly reflecting at the lasing wavelength may be used to reflect a amplified one-pass laser beam <b>389</b> back into the wedged laser active slab <b>371</b> through the window <b>379</b>, but at a slightly different angle from an output angle so that the one-pass-amplified laser beam <b>389</b> makes a second zig-zag pass <b>387</b> through the gain sheet <b>373</b>. A two-pass-amplified laser beam <b>391</b> emerges at the window <b>377</b> at a substantially different angle from the incidence angle of the incoming laser beam <b>375</b>. The two-pass-amplified laser beam <b>391</b> may be separated from the incoming laser beam <b>375</b> using, for example, a pick-off mirror <b>393</b> that is highly reflecting at the lasing wavelength. An optional lens <b>385</b> may comprise a cylinder lens or a spherical lens, for example, depending on the size of the incoming laser beam <b>375</b>. The optional lens <b>385</b> may serve to maintain mode matching for the second zig-zag pass <b>387</b> for the amplified laser beam <b>389</b> through the wedged laser active slab <b>371</b>.

In another embodiment, shown in <figref idref="DRAWINGS">FIG. 25</figref>, a laser amplifier <b>400</b> includes one or more diode pump sources (not shown) producing a gain sheet <b>405</b> in a wedged laser active slab <b>403</b> comprising a block of the laser active material as described above. The laser amplifier <b>400</b> is configured as a two-pass amplifier in which an incoming laser beam <b>401</b> enters a first lateral face <b>411</b> of the wedged laser active slab <b>403</b>, makes a first zig-zag pass <b>409</b> through the gain sheet <b>405</b>, and a two-pass-amplified laser beam <b>407</b> emerges from a second lateral face <b>413</b> of the wedged laser active slab <b>403</b> after making a second zig-zag pass <b>419</b> through the gain sheet <b>405</b>. A first optical coating <b>415</b> is patterned to have an entrance window <b>421</b>, and a second optical coating <b>417</b> is patterned to have an entrance/exit window <b>423</b> and an exit window <b>425</b>. Both the first optical coating <b>415</b> and the second optical coating <b>417</b> are highly reflecting at the laser wavelength.

The external mirror <b>383</b> reflects an one-pass-amplified laser beam <b>427</b> back into the wedged laser active slab <b>403</b> through the entrance/exit window <b>423</b> at a slightly different angle from emergence so that the one-pass-amplified laser beam <b>427</b> makes the second zig-zag pass <b>419</b> through the through the gain sheet <b>405</b> of the wedged laser active slab <b>403</b>. The external mirror <b>383</b> may be adjusted so that the two-pass-amplified laser beam <b>407</b> emerges at the exit window <b>425</b>. The optional lens <b>385</b> may be a cylinder or spherical lens, and may be utilized to maintain mode matching for the second zig-zag pass <b>419</b>.

In still another embodiment, shown in <figref idref="DRAWINGS">FIG. 26</figref>, a two-pass laser amplifier <b>430</b> includes a wedged laser active slab <b>431</b> having optical coatings <b>433</b> and <b>435</b>, highly reflecting at laser wavelength and highly transmitting at pump power wavelength, disposed on a portion of a first lateral face <b>437</b> and on a portion of a second lateral face <b>439</b>, respectively. A first window <b>441</b> and a second window <b>443</b> are provided on the first lateral face <b>437</b>, and a third window <b>445</b> and a fourth window <b>447</b> are provided on the second lateral face <b>439</b> as shown. An input laser beam <b>451</b> enters the wedged laser active slab <b>431</b> at the first window <b>441</b>, makes a first zig-zag pass <b>453</b> within a gain sheet <b>461</b>, exits at the second window <b>443</b> to follow an external path <b>455</b>, enters at the fourth window <b>445</b> to make a second zig-zag pass <b>457</b>, and exits at the third window <b>447</b> as a two-pass-amplified laser beam <b>459</b>. The gain sheet <b>461</b> may be produced by one or more diode pump sources (not shown), as described above. A first external mirror <b>463</b> and a second external mirror <b>465</b> are highly reflecting at the lasing wavelength and serve to direct lasing radiation into the wedged laser active slab <b>431</b> for the second zig-zag pass <b>457</b>. Optional lenses <b>467</b> and <b>469</b>, which can be cylindrical or spherical lenses, may be provided to maintain proper mode matching within the wedged laser active slab <b>431</b>.

In yet another embodiment, shown in <figref idref="DRAWINGS">FIG. 27</figref>, a two-pass laser amplifier <b>470</b> is configured such that an external mirror is not required. The two-pass laser amplifier <b>470</b> includes a laser active slab <b>471</b> having optical coatings <b>473</b> and <b>475</b>, highly reflecting at a wavelength of an input laser beam <b>491</b> and highly transmitting at pump power wavelength, disposed on a portion of a first lateral face <b>477</b> and on a portion of a second lateral face <b>479</b>, respectively. A first window <b>481</b> is provided on the first lateral face <b>477</b>, and a second window <b>483</b> is provided on the second lateral face <b>479</b> as shown. The input laser beam <b>491</b> enters the laser active slab <b>471</b> at the first window <b>481</b>, which may be anti-reflection coated at the wavelength of the input laser beam <b>491</b>, makes a first zig-zag pass <b>493</b> within a gain sheet <b>495</b> produced by one or more diode pump sources (not shown), reflects from a longitudinal face <b>485</b> by means of total internal reflection, makes a second zig-zag pass <b>497</b>, and exits at the second window <b>483</b> as a two-pass-amplified laser beam <b>499</b>. The longitudinal face <b>485</b> is oriented perpendicularly to a plane defined by the gain sheet <b>495</b> so that a one-pass-amplified laser beam (not shown) completing the first zig-zag pass though the gain sheet <b>495</b> remains in the gain sheet <b>495</b> after reflecting from the longitudinal face <b>485</b>.

In an alternative embodiment, shown in <figref idref="DRAWINGS">FIG. 28</figref>, a two-pass laser amplifier <b>500</b> is also configured without external mirrors. The two-pass laser amplifier <b>500</b> includes a wedged laser active slab <b>501</b> with optical coatings <b>503</b> and <b>505</b>, highly transmitting at pump power wavelength and highly reflecting at lasing wavelength, disposed on respective portions of a first lateral face <b>507</b> and a second lateral face <b>509</b>, respectively. A first window <b>511</b> is provided on the first lateral face <b>507</b>, and a second window <b>513</b> is provided on the second lateral face <b>509</b> as shown, where the windows <b>511</b> and <b>513</b> may be anti-reflection coated at the lasing wavelength. An input laser beam <b>521</b> is directed into the wedged laser active slab <b>501</b> at a specified angle of incidence to the first window <b>511</b>, and is propagated toward the narrower end of the wedged laser active slab <b>501</b> making a first zig-zag pass <b>523</b> within a gain sheet <b>515</b> produced by one or more diode pump sources (not shown).

The dihedral wedge angle formed by the first lateral face <b>507</b> and the second lateral face <b>509</b>, provides for each successive reflection between the first lateral face <b>507</b> and the second lateral face <b>509</b> to be shorter and occur at a smaller angle of incidence than a previous reflection until the direction of propagation is reversed, at a shortest path leg <b>525</b> of the first zig-zag pass <b>523</b>. The input laser beam <b>521</b> may subsequently make a second zig-zag pass <b>527</b> in the direction opposite to the first zig-zag pass <b>523</b> and may produce a two-pass-amplified laser beam <b>529</b> at the second window <b>513</b>. Depending on the angle of incidence of the input laser beam <b>521</b>, the second zig-zag pass <b>527</b> can be configured to exit the wedged laser active slab <b>501</b> at the second window <b>513</b>, as shown, or at the first window <b>511</b> at a different angle (not shown) from the input laser beam <b>521</b>. The distance from a first longitudinal face <b>517</b> to the shortest path leg <b>525</b>, that is the distance into the wedged laser active slab <b>501</b> at which the first zig-zag path <b>523</b> reverses direction, may be determined by various parameters, including: the angle of incidence of the input laser beam <b>521</b>, the magnitude of the dihedral wedge angle formed by the first lateral face <b>507</b> and the second lateral face <b>509</b>, and the width of the wedged laser active slab <b>501</b>, that is, the distance between the first lateral face <b>507</b> and the second lateral face <b>509</b>. The desired configuration and operating parameters can thus be determined by one of ordinary skill in the art. Preferably, the shortest path leg <b>525</b> is located sufficiently close to a second longitudinal face <b>519</b> that good spatial overlap is realized for the first zig-zag pass <b>523</b> and for the second zig-zag pass <b>525</b>, with the gain sheet <b>515</b>.

The number of passes realized in a laser amplifier having a wedged laser active slab is not limited to one or two, as can be explained with reference to a three-pass laser amplifier <b>530</b>, shown in <figref idref="DRAWINGS">FIG. 29</figref>, and a four-pass laser amplifier <b>570</b>, shown in <figref idref="DRAWINGS">FIG. 30</figref>. In <figref idref="DRAWINGS">FIG. 29</figref>, the three-pass laser amplifier <b>530</b> includes a wedged laser active slab <b>531</b> with optical coatings <b>533</b> and <b>535</b>, highly transmitting at pump power wavelength and highly reflecting at lasing wavelength, disposed on respective portions of a first lateral face <b>537</b> and a second lateral face <b>539</b>, respectively. A first window <b>541</b> and a second window <b>543</b> are provided on the first lateral face <b>537</b>, and a third window <b>545</b> and a fourth window <b>547</b> are provided on the second lateral face <b>539</b> as shown. An input laser beam <b>551</b> is directed into the wedged laser active slab <b>531</b> at a specified angle of incidence to the first window <b>541</b> to make a first zig-zag pass <b>553</b> within a gain sheet <b>549</b> produced by one or more diode pump sources (not shown).

A first external mirror <b>561</b>, highly reflecting at the lasing wavelength, reflects a one-pass-amplified laser beam <b>553</b><i>a </i>from the first zig-zag pass <b>553</b> back into the wedged laser active slab <b>531</b> through the second window <b>543</b>, but at a slightly different angle from emergence, so that a second zig-zag pass <b>555</b> is made through the gain sheet <b>549</b>. Preferably, the first external mirror <b>561</b> is adjusted so that the second zig-zag pass <b>555</b> is incident upon the third window <b>545</b>. A second external mirror <b>563</b>, also highly reflecting at the lasing wavelength, reflects a two-pass-amplified laser beam <b>555</b><i>a </i>from the second zig-zag pass <b>555</b> back through the third window <b>545</b> so that a third zig-zag pass <b>557</b> can be made through the gain sheet <b>549</b>. By adjusting the second external mirror <b>563</b>, the third zig-zag pass <b>557</b> can be made incident on the fourth window <b>547</b>, as shown, or can alternatively be made incident on the second window <b>543</b> (not shown). Optional lenses <b>565</b> and <b>567</b> may be cylindrical or spherical lenses, depending on the size of the input laser beam <b>551</b>, and may be needed to maintain mode matching for the second zig-zag pass <b>555</b> and for the third zig-zag pass <b>557</b>.

The four-pass laser amplifier <b>570</b>, shown in <figref idref="DRAWINGS">FIG. 30</figref>, includes a wedged laser active slab <b>571</b> with optical coatings <b>573</b> and <b>575</b>, highly transmitting at pump power wavelength and highly reflecting at a wavelength of an input laser beam <b>579</b>, disposed on respective portions of a first lateral face <b>577</b> and a second lateral face <b>579</b>, respectively. This configuration provides for a first window <b>581</b> and a second window <b>583</b> on the first lateral face <b>577</b>, and for a third window <b>585</b> and a fourth window <b>587</b> provided on the second lateral face <b>579</b> as shown. The windows <b>581</b>, <b>583</b>, <b>585</b>, and <b>587</b> may be anti-reflection coated at a wavelength of the input laser beam <b>591</b>. The input laser beam <b>591</b> is directed into the wedged laser active slab <b>571</b> at a specified angle of incidence to the first window <b>581</b> to make a first zig-zag pass <b>593</b> within a gain sheet <b>589</b> produced by one or more diode pump sources (not shown).

A first external mirror <b>603</b><i>a</i>, highly reflecting at the lasing wavelength, reflects a one-pass-amplified laser beam <b>593</b><i>a </i>from the first zig-zag pass <b>593</b> to a second external mirror <b>603</b><i>b </i>and back into the wedged laser active slab <b>571</b> through the third window <b>585</b>, so that a second zig-zag pass <b>595</b> is made through the gain sheet <b>589</b>. Preferably, the first and second external mirrors <b>603</b><i>a </i>and <b>603</b><i>b </i>are adjusted so that the second zig-zag pass <b>595</b> terminates at the fourth window <b>587</b>. A third external mirror <b>605</b>, also highly reflecting at the lasing wavelength, reflects a two-pass-amplified laser beam <b>595</b><i>a </i>from the second zig-zag pass <b>595</b> back through the third window <b>587</b>, but at a slightly different angle from emergence, so that a third zig-zag pass <b>597</b> is made through the gain sheet <b>589</b>. By suitable adjustment, the third external mirror <b>605</b> can direct the terminus of the third zig-zag pass <b>597</b> onto the third window <b>585</b>, as shown.

The second external mirror <b>603</b><i>b </i>and the first external mirror <b>603</b><i>a </i>reflect a three-pass-amplified laser beam <b>597</b><i>a </i>from the third zig-zag pass <b>597</b> back into the wedged laser active slab <b>571</b> through the second window <b>583</b>, so that a fourth zig-zag pass <b>599</b> is made through the gain sheet <b>589</b> and a four-pass-amplified laser beam <b>601</b> is output from the first window <b>581</b>. Preferably, the first and second external mirrors <b>603</b><i>a </i>and <b>603</b><i>b </i>are adjusted so that the terminus of the fourth zig-zag pass <b>599</b> terminates at the first window <b>581</b>. Optional lenses <b>607</b><i>a </i>and <b>607</b><i>b </i>may be cylindrical or spherical lenses, depending on the size of the input laser beam <b>591</b>, and may be needed to maintain mode matching for the second zig-zag pass <b>595</b> and for the fourth zig-zag pass <b>599</b>. Optional lens <b>609</b> may be a cylindrical or a spherical lens and may be needed to maintain mode matching for the third zig-zag pass <b>597</b>.

It can be appreciated by one skilled in the relevant art that any of the above one-pass or multi-pass laser amplifier embodiments may include one or more bulk Bragg gratings, also referred to in the relevant art as volume Bragg grating micro-optics or 3-D Bragg gratings. The volume Bragg grating serves to narrow the spectral bandwidth of emission from high-power diode lasers and diode laser bars, such as the sources of pump power described above, and the volume Bragg grating further stabilizes the peak diode laser emission wavelength as a function of temperature, and may advantageously reduce laser system cooling requirements.

A laser amplifier <b>620</b>, shown in <figref idref="DRAWINGS">FIG. 31</figref>, includes a laser active slab <b>621</b>, such as any one of the laser active slabs <b>11</b>, <b>81</b>, <b>111</b>, <b>151</b>, <b>181</b>, <b>201</b>, <b>221</b>, <b>251</b>, <b>261</b>, <b>331</b>, <b>361</b>, <b>371</b>, <b>403</b>, <b>431</b>, <b>471</b>, <b>501</b>, <b>531</b>, or <b>571</b> comprising laser active material as described above. In the configuration shown, the laser amplifier <b>620</b> includes a first diode bar <b>623</b> with a first microlens <b>627</b>, and an optional second diode bar <b>625</b> with an optional second microlens <b>629</b>. A first volume Bragg grating <b>631</b> may be disposed between the first microlens <b>627</b> and a first lateral side <b>635</b> of the laser active slab <b>621</b>, and an optional second volume Bragg grating <b>633</b> may be disposed between the second microlens <b>629</b> and a second lateral side <b>637</b> of the laser active slab <b>621</b> as shown. Volume Bragg gratings may be commercially available from Ondax, Inc. of Monrovia, Calif. and from PD-LD, Inc. of Pennington, N.J., for example.

In an alternative embodiment, shown in <figref idref="DRAWINGS">FIG. 32</figref>, a laser amplifier <b>640</b> includes a laser active slab <b>641</b> pumped from longitudinal faces rather than from lateral faces as described above. The dihedral wedge angle of the laser active slab <b>641</b> has been exaggerated for clarity of illustration. The laser amplifier <b>640</b> may include a first diode bar <b>643</b> with a first microlens <b>647</b> disposed adjacent a first longitudinal face <b>651</b>, and an optional second diode bar <b>645</b> with a second microlens <b>649</b> disposed adjacent a second longitudinal face <b>653</b> as shown. A first longitudinal optical coating <b>655</b> may be disposed on the first longitudinal face <b>651</b> and a second longitudinal optical coating <b>657</b> may be disposed on the second longitudinal face <b>653</b>.

The longitudinal optical coatings <b>655</b> and <b>657</b> are highly transmitting to the wavelength of pump power in the diode bars <b>643</b> and <b>645</b>. A first lateral optical coating <b>671</b> may be disposed on a first lateral face <b>675</b> and a second lateral optical coating <b>673</b> may be disposed on a second lateral face <b>677</b>, where the lateral optical coatings <b>671</b> and <b>673</b> are highly reflecting to the wavelength of lasing radiation. In the single-pass configuration shown, a first window <b>681</b> is provided for an incoming laser beam (not shown), and a second window <b>683</b> is provided for an emitted amplified laser beam (not shown). It should be understood that, in an alternate configuration, an exit window (not shown) may be provided on the second lateral face <b>677</b>.

In still another embodiment, shown in <figref idref="DRAWINGS">FIG. 33</figref>, a laser amplifier <b>690</b> includes a laser active slab <b>691</b>, a first diode bar <b>693</b> with a first microlens <b>697</b> and an optional second diode bar <b>695</b> with a second microlens <b>699</b>. The first diode bar <b>693</b> and the optional second diode bar <b>695</b> may be used to produce a gain sheet <b>709</b> in the laser active slab <b>691</b>. The laser active slab <b>691</b> includes a first convex lateral side <b>701</b> with a first optical coating <b>705</b>, and a second convex lateral side <b>703</b> with a second optical coating <b>707</b>. The optical coatings <b>705</b> and <b>707</b> are highly transmitting at the wavelength of the pump radiation produced by the diode bars <b>693</b> and <b>695</b> and are highly reflecting at lasing wavelength.

In another application of the present invention, shown in <figref idref="DRAWINGS">FIG. 34</figref>, a laser oscillator configuration system <b>710</b> includes a laser active slab <b>711</b>, a first diode bar <b>713</b> with a first microlens <b>717</b> and a second diode bar <b>715</b> with a second microlens <b>719</b>. A first mirror <b>721</b> comprises an output coupler mirror that is partially transmitting at the laser wavelength. A second mirror <b>723</b> is highly reflecting at the laser wavelength. The mirrors <b>721</b> and <b>723</b> form a laser cavity <b>720</b> with the diode-pumped laser active slab <b>711</b>. The mirrors <b>721</b> and <b>723</b> are aligned such that a resonator axis <b>725</b> follows a zig-zag path <b>727</b> within the laser active slab <b>711</b>. The zig-zag path <b>727</b> may originate at a first window <b>737</b> and terminate at a second window <b>739</b>, where the windows <b>737</b> and <b>739</b> are highly transmitting to a lasing wavelength. In an alternative embodiment (not shown) the second window <b>739</b> may be located on the same lateral face of the laser active slab <b>711</b>. The laser cavity <b>720</b> may also be known as a linear or standing-wave resonator or oscillator in the relevant art. Optional lenses <b>731</b>, <b>733</b>, and <b>735</b> may be spherical lenses and/or cylinder lenses, and may be required to achieve proper mode matching of the laser resonator mode to a gain sheet <b>729</b> in the diode-pumped laser active slab <b>711</b>. Additional intra-cavity elements (not shown) may be included in the laser cavity <b>720</b> to achieve Q-switched or mode-locked operation of the oscillator, and/or to perform intra-cavity nonlinear optical wavelength conversion, as is well known in the art.

In still another application of the present invention, shown in <figref idref="DRAWINGS">FIG. 35</figref>, a laser oscillator configuration system <b>740</b> includes a laser active slab <b>741</b>, a first diode bar <b>743</b> with a first microlens <b>747</b> and a second diode bar <b>745</b> with a second microlens <b>749</b>. A first mirror <b>751</b>, a second mirror <b>753</b>, and a third mirror <b>755</b> are positioned and aligned as shown with respect to the laser active slab <b>741</b> so as to form a laser cavity having a resonator axis <b>757</b> that follows a zig-zag path <b>759</b> within the laser active slab <b>741</b>. The first mirror <b>751</b> comprises an output coupler mirror that is partially transmitting at the lasing wavelength. The mirrors <b>753</b> and <b>755</b> are highly reflecting at the lasing wavelength.

The configuration formed is typically referred to in the relevant art as a “ring” resonator or a ring oscillator. The optional lenses <b>751</b>, <b>753</b>, and <b>755</b> may be spherical lenses and/or cylinder lenses, and may be required to achieve proper mode matching of the laser resonator mode to a gain sheet <b>761</b> in the diode-pumped laser active slab <b>741</b>. Additional intra-cavity elements (not shown) may be included in the ring laser oscillator configuration system <b>740</b> to achieve Q-switched or mode-locked, to force unidirectional ring laser operation, and/or to perform intra-cavity nonlinear optical wavelength conversion, as is well known in the art.

In yet another application of the present invention, shown in <figref idref="DRAWINGS">FIG. 36</figref>, an oscillator-amplifier “chain” <b>800</b> includes an oscillator <b>801</b>, a first laser amplifier stage <b>770</b>, and a second laser amplifier stage <b>780</b> producing an amplified laser beam <b>809</b>. The oscillator <b>801</b> may comprise a standing-wave laser oscillator utilizing one or more of the diode-pumped laser active slabs <b>11</b>, <b>81</b>, <b>111</b>, <b>151</b>, <b>181</b>, <b>201</b>, <b>221</b>, <b>251</b>, <b>261</b>, <b>331</b>, <b>361</b>, <b>371</b>, <b>403</b>, <b>431</b>, <b>471</b>, <b>501</b>, <b>531</b>, or <b>571</b> described above. In an alternative embodiment, the oscillator <b>801</b> may comprise a ring laser oscillator, such as disclosed in the ring laser oscillator configuration system <b>740</b> shown in <figref idref="DRAWINGS">FIG. 35</figref>.

Alternatively, the oscillator <b>801</b> can be of another oscillator design that emits a laser beam having a wavelength that is matched at the gain or lasing wavelength of the subsequent amplifier stages that incorporate one or more of the diode-pumped laser active slabs <b>11</b>, <b>81</b>, <b>111</b>, <b>151</b>, <b>181</b>, <b>201</b>, <b>221</b>, <b>251</b>, <b>261</b>, <b>331</b>, <b>361</b>, <b>371</b>, <b>403</b>, <b>431</b>, <b>471</b>, <b>501</b>, <b>531</b>, or <b>571</b> described above. Such alternative laser oscillators may include, for example, a microchip laser, a fiber laser, or a semiconductor diode laser, including CW, Q-switched laser, gain-switched laser, and mode-locked versions of these lasers, but the present invention is not limited to the use of such laser oscillators.

The first laser amplifier stage <b>770</b> may include a laser active slab <b>771</b>, a first diode bar <b>773</b> with a first microlens <b>777</b>, and a second diode bar <b>773</b> with a second microlens <b>779</b>. The second laser amplifier stage <b>780</b> may include a laser active slab <b>781</b>, a first diode bar <b>783</b> with a first microlens <b>787</b>, and a second diode bar <b>783</b> with a second microlens <b>789</b>. The second laser amplifier stage <b>780</b> may be of the same type and configuration as the first laser amplifier stage <b>770</b>.

An oscillator output beam <b>803</b> may pass through a first mode-matching component <b>805</b><i>a </i>before entering the first laser amplifier stage <b>770</b>. The first mode-matching component <b>805</b><i>a </i>serves to adjust the oscillator beam size and shape in each laser amplifier stage to optimally extract power and/or energy from the respective laser amplifier stage. The first mode-matching component <b>805</b><i>a </i>may comprise an image relay lens, or a lens system, that performs one-to-one (i.e., 1:1) imaging from the laser active slab <b>771</b> to the laser active slab <b>781</b>, for example.

The oscillator output beam <b>803</b> may be directed into the first laser amplifier stage <b>770</b> via a first mirror <b>807</b><i>a</i>, which is highly reflecting at the laser wavelength. The output beam <b>803</b> follows a zig-zag beam path in the first laser amplifier stage <b>770</b> and in the second laser amplifier stage <b>780</b>. A second mode-matching component <b>805</b><i>b </i>may be provided between the first laser amplifier stage <b>770</b> and the second laser amplifier stage <b>780</b>. It should be understood that, although the diagram shows single-pass configurations for the first laser amplifier stage <b>770</b> and the second laser amplifier stage <b>780</b>, the invention is not so limited and the chain laser amplifier <b>800</b> may comprise one or more two-pass, three-pass, or four-pass laser amplifier configurations, as described in greater detail above. Alternatively, the laser amplifier chain <b>800</b> can be extended with one or more additional amplifier stages, such as a third amplifier stage <b>790</b>, where the amplified laser beam <b>809</b> may be directed into the third amplifier stage <b>790</b> by a third mirror <b>791</b>. In an alternative embodiment, the laser amplifier chain <b>800</b> may include a Faraday isolator <b>802</b>, disposed in the optical path of the oscillator output beam <b>803</b>, between the oscillator <b>801</b> and the first mode-matching component <b>805</b><i>a. </i>

In another alternative application of the present invention, shown in <figref idref="DRAWINGS">FIG. 37</figref>, an amplified spontaneous emission source <b>810</b> includes a wedged laser active slab <b>811</b>, a first diode pump source <b>821</b> with a microlens <b>823</b>, and a second diode pump source <b>825</b> with a microlens <b>827</b>. The wedged laser active slab <b>811</b> may include optical coatings <b>813</b> and <b>815</b>, highly transmitting at pump power wavelength and highly reflecting at lasing wavelength, with a first window <b>817</b> and a second window <b>819</b>. A spontaneous emission <b>831</b> originating in the wedged laser active slab <b>811</b> makes a first zig-zag path <b>833</b> and exits at the first window <b>817</b> as amplified spontaneous emission radiation <b>835</b> incident on a mirror <b>829</b>. The mirror <b>829</b> is highly reflecting at the lasing wavelength of the active material contained in the wedged laser active slab <b>811</b>, and serves to reflect the amplified spontaneous emission radiation <b>835</b> back into the wedged laser active slab <b>811</b> via the first window <b>817</b>. The amplified spontaneous emission radiation <b>835</b> makes a second zig-zag path <b>837</b> through the wedged laser active slab <b>811</b> to emerge as an amplified spontaneous emission output beam <b>839</b>.

In a preferred embodiment, the diode pump power is substantial enough, the lengths of the zig-zag passes <b>833</b> and <b>837</b> in the laser active slab <b>811</b> are long enough, and the diode-pumped gain-per-unit-length, g_o, is adequately high, such that the two zig-zag passes <b>833</b> and <b>837</b> alone are sufficient to extract power from the laser active slab <b>811</b> with good efficiency. Other amplified spontaneous emission source configurations are also possible in which the spontaneous emission makes three or four passes through the laser active slab, as described above, as might be appropriate when using lower-gain laser active materials in the amplified spontaneous emission source.

<figref idref="DRAWINGS">FIG. 38</figref> is a diagrammatical illustration of a laser amplifier <b>840</b> having a laser active slab <b>841</b> with undoped lateral sections <b>843</b> and <b>845</b>. The configuration of the laser active slab <b>841</b> may improve the quality of a laser beam <b>851</b> or resonator mode being amplified under operating conditions in which diode pump power from diode bars <b>857</b> and <b>859</b> is sufficiently high enough to produce thermo-mechanical distortion at lateral sides <b>847</b> and <b>849</b>. The undoped lateral sections <b>843</b> and <b>845</b> comprise the same active material, or a similar host material, as the laser active slab <b>841</b> but do not include the laser active ions present in the laser active slab <b>841</b>. As is well-known to someone of ordinary skill in the art, such use of an undoped material section bonded to a doped laser crystal (e.g., a slab or a rod) may serve to reduce thermo-mechanical distortion of a diode-pumped face, and thereby increases the pump power density limit at which thermal fracture of the diode-pumped face may occur. The undoped lateral sections <b>843</b> and <b>845</b> may include respective film coatings <b>853</b> and <b>855</b> highly transmitting at pump wavelength and highly reflecting at lasing wavelength. The undoped lateral sections <b>843</b> and <b>845</b> may be optically contacted and bonded to the laser active slab <b>841</b> using diffusion bonding techniques, or other epoxy-less methods, as have become commercially available in recent years from suppliers such as VLOC Corporation of New Port Richey, Fla. and Onyx Optics of Dublin, Calif.

While the invention has been described with reference to particular embodiments, it will be understood that the present invention is by no means limited to the particular constructions and methods herein disclosed and/or shown in the drawings, but also comprises any modifications or equivalents within the scope of the claims.